1. Field
One or more embodiments relate to a piezoelectric micro speaker and manufacturing a piezoelectric micro speaker.
2. Description of the Related Art
As terminals for personal voice communication and data communication are rapidly developed, the available amount of data to be transferred and received is continuously increasing, while the terminals are required to be small and multi-functional.
In order to satisfy this requirement, research has been recently conducted on an acoustic device using micro electro-mechanical system (MEMS) technology. In particular, a micro speaker using MEMS and semiconductor technologies allows a mass production of the micro speaker with a small size and low cost and is easily integrated with a peripheral circuit.
Micro speakers using MEMS technology are mainly classified as an electrostatic type, an electromagnetic type, and a piezoelectric type. In particular, a piezoelectric micro speaker may be driven by using a voltage that is lower than a voltage used in an electrostatic micro speaker and may be formed to be simpler and slimmer than an electromagnetic micro speaker.
A general piezoelectric micro speaker has a structure in which a piezoelectric actuator including a piezoelectric layer formed between two electrode layers is stacked on a surface of a diaphragm, and generates sound when a voltage is applied to the piezoelectric layer via the two electrode layers, deforming the piezoelectric layer, and thus vibrating the diaphragm.
A wiring process for applying a voltage and a packaging process for protecting the diaphragm are required to install the piezoelectric micro speaker in a system. In the packaging process, a front plate in which a radiation hole radiating sound is formed is bonded on a front surface of a device plate on which the diaphragm and the piezoelectric actuator are formed, and a rear plate in which a bent hole suppressing a vibration damping effect and tuning sound characteristics is formed is bonded on a rear surface of the device plate.
However, sound generated by vibrating the diaphragm is radiated forward and backward. The sound radiated backward is reflected forward by the rear plate due to an acoustic impedance difference. Due to an impedance difference between air and silicon (Si) forming the rear plate, most of the sound radiated backward is reflected. The reflected sound disturbs the vibration of the diaphragm and causes an interference having a phase difference from the sound radiated forward from the diaphragm, thereby reducing the sound pressure and distorting sound characteristics.